1. Field of the Invention
The present invention relates to a method and equipment for establishing a reliable data link also in a data transfer environment with strong interference.
2. Description of the Prior Art
In telephone networks of poor quality, and especially in cellular networks, data transfer connections must endure various disturbances caused by the network. These disturbances vary from connection to connection and may also vary during a connection, especially when one of the participants in the connection is a mobile station moving from one place to another.
This kind of functional environment makes great demands on the interference tolerance of the modem. In some cases, the present modems make data links rather unreliable and slow, thus forming a great obstacle on the development of personal communicators, especially in mobile use.
Connection establishment in fixed networks is simpler than in cellular networks because the fixed network environment is fairly constant. Due to this systems primarily designed for fixed networks are unable to remedy/mend problems inherent to cellular networks.
Data transfer in cellular networks is a fairly new technological concept, and modems designed for cellular use have been on market for only a short period of time. While developing cellular modems, the usual approach has been to fulfil the functional requirements and, accordingly, new problems brought on by the radio line used in data transfer have been ignored to some degree. It has been erroneously assumpted that merely the usage of an error correcting protocol (such as CCITT V.42) is sufficient to guarantee a reliable data link. Thus manufacturers have used for cellular modems principally the same programs and methods as for modems in fixed networks, for example for the changing of data transfer rate and for other corresponding tasks.
The conventional PID (Proportional Integral Derivative) control systems of the known technology hare fixed limits to the values of each input parameter which do not take in account the interactions caused by the disturbances of the cellular network. Further, these PID control systems cannot adapt to the changing environmental circumstances, but the system must be adjusted to correspond with the changed circumstances, for example by modifying the modem software in accordance with the usage in question. It is not possible for the user to tune the modem for different operating environments. Further, the radio connection may suffer from strong burst interference which may lead to a premature disconnection, although it could have been possible to maintain the connection by repeating the handshake/synchronization after the error burst.
When establishing a connection, the known modems are unable to use statistical information, such as telephone numbers, estimated length of the connection, or selection of previous connections. So far it has been desirable to maintain the rate control with the current data transfer connection as simple as possible, because the modem processor control has already been used nearly to its full capacity. Usage of more efficient processors also enables the usage of more efficient adaptive algorithms for environmental modifications without degrading the basic modem operation.
Some modems offer the user limited possibilities to control the connection establishment for example in such a way that he can choose the desired transfer rate and activate certain handshaking methods, when it is assumed that the user makes the needed modifications in the registers. However, for most users this task is too complicated. Other modems offer no such possibilities.